This invention relates to a method and apparatus for preventing errors in PCM signal processing apparatus and, more particularly, to such a method and apparatus which is particularly adapted for use with PCM signal processing apparatus which receives PCM signals in a time-interleaved error-correction code from either of two different data sources.
Recently, digital techniques have been used for the transmission and recording of audio signals. For example, a rotary-head type video tape recorder (VTR), having a high recording density, can be used to record pulse code modulated (PCM) signals representing audio information. However, when a PCM-encoded signal is recorded and subsequently reproduced, the possibility exists that noise, interference, signal dropout, and the like may be present so as to destroy some of the reproduced PCM signals. Such loss of data may result in serious errors in the reproduced signal so as to interfere with satisfactory audio reproduction.
In order to minimize this problem of signal loss, error-correction codes have been proposed for use in encoding the PCM signals prior to recording or transmission. By using such error-correction codes, erroneous PCM signals which are reproduced or received may be corrected or compensated so as to avoid the aforenoted interference in audio reproduction.
One advantageous error-correction code which has been proposed for such PCM signals is the so-called time-interleaved code, some examples of which are described in copending application Ser. No. 86,677, filed Oct. 19, 1979 and now U.S. Pat. No. 4,306,305, and in copending application Ser. No. 195,625, filed Oct. 9, 1980, both assigned to the assignee of the present invention. Generally, in the time-interleaved error-correction code, plural channels of PCM signals are produced, each channel being constituted by a sequence, or series, of successive PCM words. These plural channels may be derived from an analog-to-digital converter which is used to digitize an input analog audio signal, such as a stereophonic signal. A data block is formed of one word in each channel, which words, typically, appear in parallel-by-word format. These parallel-appearing words are used to derive one or more error-correction words, such as parity words. Then, each PCM word in the data block, as well as the error-correction word (or words) is delayed by a respectively different time delay so as to effectively time-interleave the PCM and error correction words. These time-interleaved words, which are present in parallel-by-word form, are supplied, concurrently, to an error-detection word generator, such as a cyclic redundancy code (CRC) generator so as to produce an error detection word. This error detection word is combined with the time-interleaved PCM and error-correction words so as to form a time-interleaved transmission block. The time-interleaved transmission block then may be recorded, transmitted, or otherwise utilized.
When the time-interleaved transmission block is reproduced, or received, the various interleaved words, together with the error-detection word, are examined to determine if an error is present in this particular transmission block. Error detection codes, such as the CRC code, are well-known for providing this error-detection feature. If an error is detected in this transmission block, all of the interleaved PCM and error-correction words are identified as being erroneous, irrespective of whether each such word is, in fact, in error or correct. Then, these time-interleaved, identified, PCM and error-correction words are time de-interleaved so as to reconstruct the original data block. If a de-interleaved PCM word is identified as being erroneous, it can be corrected, by conventional error-correction techniques, such as by parity decoding, provided that none of the other words included in the same block are erroneous. If the reconstructed, de-interleaved block includes two error-correction words, then two erroneous PCM words included in that de-interleaved block can be corrected. The foregoing techniques are described more particularly in the aforementioned, copending applications.
By using the aforementioned time-interleaved encoding technique, effects due to a so-called burst error are minimized. The expression "burst error" generally refers to an error interval, wherein recorded or transmitted data is dropped out, that extends over a period of time sufficient to encompass a plurality of time-interleaved transmission blocks. However, even if all of the PCM and error-correction words included in a number of time-interleaved transmission blocks are distorted, upon reconstructing the original, de-interleaved transmission blocks, it is expected that, generally, only one word in the reconstructed block is distorted. That is, the time-interleaved encoding technique serves to disperse a burst error throughout many reconstructed blocks. Then, since only a single word in a reconstructed, de-interleaved block is erroneous, such errors may be corrected or compensated by conventional error-correction or compensation techniques.
It is advantageous to utilize PCM signal processing apparatus including the aforementioned time-interleaved encoder/decoder as an adapter to be quickly and simply connected to a VTR such that a conventional VTR may be used to record PCM-encoded audio signals. It also is advantageous to use this type of encoder/decoder to receive PCM signals from various sources. For example, the PCM decoder may be connected through a switching arrangement either to the playback section of a VTR or to the output of a PCM encoder. Depending upon the condition of the switch, time-interleaved transmission blocks are supplied to the decoder from one (the VTR) or the other (the PCM encoder) data source. Of course, since the time-interleaved transmission blocks supplied by both sources exhibit the same format, the decoder functions to decode the received transmission blocks, regardless of the particular source from which they are transmitted, and to reconstruct the original audio signals. In many instances, it may be desirable by the user of the apparatus to change over from one source (e.g. the VTR) to the other. During the transition interval, which exists for a finite time, the decoder effectively is supplied with erroneous transmission blocks. When these erroneous transmission blocks are time de-interleaved, in accordance with the usual procedure, a number of de-interleaved blocks, commencing with the first de-interleaved block at the beginning of the transition interval, contain some PCM and/or error-correction words derived from one source and other PCM and/or error-correction words derived from the other. Furthermore, one or more of the words included in such de-interleaved blocks are erroneous because they have been de-interleaved from those transmission blocks which were supplied during the transition interval. Nevertheless, if one of these PCM words in the de-interleaved block is erroneous, the error-corrector of the decoder will attempt to operate in its usual manner to correct this erroneous PCM word. However, it is possible that the erroneous word may be derived from one source whereas other words included in this de-interleaved block are derived from the other source. Typically, the error-correction operation attempts to reconstruct the erroneous PCM word by utilizing the error-correction word in conjunction with the remaining non-erroneous PCM words. If all of these words are derived from the same data source, there is no difficulty in reconstructing the correct PCM word. However, when some of the words are derived from one source and others are derived from the other source, there is no correlation therebetween, and the erroneous word cannot be reconstructed.
When the time-interleaved encoder/decoder of the aforementioned type is used in the example just described, the error-corrector therein attempts to "correct" the erroneous PCM word, even though such a "correction" cannot be carried out. As a result thereof, the "corrected" word is in error and, when converted back to analog form and reproduced by, for example, a loudspeaker, results in an undesired sound. This sound is disturbing and, preferably, should be avoided.
While the aforementioned undesired noise can be muted merely by carrying out a conventional muting operation whenever a change-over operation is carried out from one data source to another, this requires sensing the change-over operation. An additional, special connection must be made to, for example, the VTR, in order to derive a control signal therefrom which can be used to control the muting operation. Since such a control signal normally is not provided, this would require a special reconstruction of the VTR, which is not desired. Moreover, the production of a muting control signal frustrates the attempt to provide the PCM encoder/decoder as a mere "adapter", without special connections.